1. Field of the Invention
The embodiments discussed herein relate to a semiconductor device.
2. Background of the Related Art
Highly efficient and low-noise power converters are developed for power conditioners for photovoltaic power generation, motor controllers for electric vehicles, and the like. The power converters include inverter units formed by combining semiconductor devices each including semiconductor chips.
Insulated gate bipolar transistors (IGBT), power metal-oxide-semiconductor field-effect transistors (MOSFET), and free wheeling diodes (FWD) are used as semiconductor chips included in semiconductor devices.
FIG. 11 is a sectional view which illustrates an example of the structure of a conventional semiconductor device.
A semiconductor device 100 illustrated in FIG. 11 includes two semiconductor chips 101 and 102. The semiconductor device 100 includes an insulating substrate 103. The insulating substrate 103 includes a ceramic board 103a, a circuit board 103b stuck on a front (upper surface in FIG. 11) of the ceramic board 103a, and a metal board 103c stuck on a back (under surface in FIG. 11) of the ceramic board 103a. 
The semiconductor chips 101 and 102 are joined to the circuit board 103b by the use of solder 104 and a plurality of external terminals 105 are joined to the circuit board 103b by the use of the solder 104.
Furthermore, a heat radiation base plate 107 is joined to the back of the metal board 103c by the use of solder 108.
The semiconductor chips 101 and 102 are electrically connected by a bonding wire 109 and the semiconductor chip 102 and the circuit board 103b are electrically connected by the bonding wire 109.
Furthermore, the base plate 107, the insulating substrate 103, and the semiconductor chips 101 and 102 are housed in a box-shaped resin case 110 having an open bottom. Furthermore, resin is injected into the resin case 110 and is cured. By doing so, the internal components are sealed.
An inverter unit is formed by combining a plurality of semiconductor devices each of which is similar to the above semiconductor device. It is suggested that at this time the plurality of semiconductor devices are connected by connecting their external terminals by the use of plate-shaped bus bars (see, for example, International Publication Pamphlet No. WO2013/146212).
By the way, with the conventional semiconductor device 100 wiring is performed by the use of thin bonding wires. This makes it difficult to reduce the wiring inductance in the semiconductor device 100. As a result, it is impossible to accommodate high-speed switching.
In addition, with the conventional technique described in, for example, International Publication Pamphlet No. WO2013/146212, plate-shaped bus bars are used for connecting a plurality of semiconductor devices. This reduces the wiring inductance among the semiconductor devices. However, there are various demands for the shapes of packages of actual semiconductor devices. As a result, wiring routing performed by the use of bus bars often becomes complex. Furthermore, usually a bus bar becomes thinner with a reduction in the size of semiconductor devices. In many cases, this leads to an increase in inductance.